Performance study of a prototype pure CsI calorimeter for the KOTO experiment

The performance of the CsI electromagnetic calorimeter to be used for the J-PARC E14 KOTO experiment for a rare decay K L → π 0 ν ν ¯ was studied by using a positron beam from an accelerator. The energy, timing, and position resolutions for energies up to 800 MeV were measured. We also developed a new method to reconstruct the incident angle of photons based on the shower shape recorded with the calorimeter, and evaluated the method to suppress possible backgrounds for K L → π 0 ν ν ¯ decay signals.